1. Field of the Invention
This invention relates to photographic cameras having two range type automatic exposure control usuable with a motor drive unit, and more particularly to a system for selectively operating either of the shutter preselection automatic exposure control range and the diaphragm preselection automatic exposure control range with respect to the mode of operation of the motor drive unit.
2. Description of the Prior Art
Recently developed multi-purpose single lens reflex cameras are provided with a wide variaty of automatic exposure control modes with the so-called TTL type metering aspect. Some of these cameras are provided with a shutter preselection automatic exposure range alone wherein an effective diaphragm aperture value is determined based on the preselected shutter speed, and some with a diaphragm preselection automatic exposure range along wherein an effective exposure time is determined based on the preselected diaphragm value. Which of these ranges is preferred to select is dependent upon the given photographic situation. As far as such TTL-AE type of camera is concerned, therefore, it is ideal to operate these two exposure control ranges in a selective manner by simple manipulation. In this connection, various systems have been proposed for selectively controlling the operation of a camera having two range type automatic exposure control with the TTL-type metering aspect. Most recently important technological advances are directed to the use of a motor drive unit in combination with the TTL-AE camera having two operating ranges, as such motor driven camera has found an increasing demand particularly from newspaper photographers and expert armature photographers.
In application to continuous photography, it is suited to select a camera having the shutter preselection automatic exposure control range alone for use with the motor drive unit because shooting control is of primary importance. The prior art is, however, to provide some of the dual operating range AE type cameras as adapted to be usuable with a motor drive unit regardless of the fact that, in the diaphragm preselection automatic exposure range, the frame frequency is affected by the level of brightness of a scene being photographed.
In controlling the operation of this dual type camera associated with a motor drive unit which is set in RUN position to make a series of successive frame exposures, the termination of a first frame exposure is instantaneously followed by the initiation of film winding operation of which the termination is then followed by the next shutter release for making a second frame exposure. In this case, the time period necessary to complete one cycle of exposure operation is equal to the sum of the exposure time and the time interval during which the film winding mechanism is operated to advance film through the length of one frame. The frame frequency is defined as the reciprocal number of this time period in second. Now assuming that the film winding-up time period can be maintained constant from frame to frame, as each of the successive frame exposures is made with a once preselected shutter speed, the frame frequency in the shutter preselection automatic exposure range is unsusceptible to the variation of the scene brightness level. In the diaphragm preselection automatic exposure range, however, the actual exposure time is varied from frame to frame as the scene brightness level is varied. To avoid this, it is known to provide a frame frequency selector as incorporated in the motor drive unit and arranged to cooperate with the exposure control apparatus of the camera in such a manner as to establish an AND gate output. If a low frame frequency is selected, it is possible to make almost constant the frame frequency even in the diaphragm preselection mode, but significant numbers of photographic sistuations are encountered wherein such low frame frquency is unsuitable for recording purposes.
Since the subject to be taken with motor driven photographic cameras are usually in motion and the camera-to-subject distance changes permanently while shooting, the shutter preselection automatic exposure range is suitable which permits selection of relatively fast shutter speeds, though it is impossible to take into account so freely the depth of field of the subject as in the diaphragm preselection automatic exposure range.
The conventional cameras of the dual exposure range type adapted to be usuable with the motor drive unit are generally characterized by the lack of the provision for preventing accidental setting of the camera in the diaphragm preselection mode when the motor drive unit is used to cooperate with the camera, thus giving a disadvantage of making a series of successive frame exposures at ever-varying frame frequencies to limit the percentage of resultant photographs which will be found acceptable.